MIDI is not sound. You can’t simply press the Bounce button on an output object, and expect the external synth’s sound to magically turn up in your final song. MIDI is a series of commands to your MIDI devices, which then may produce the desired sound/audio. In order to have this sound included when you bounce the song to a final audiofile, you can do two things. First of all, feed the sounding results of the synths – via an external mixer or directly – into audio inputs of your soundcard. Then:
You record the MIDI tracks as audio tracks. To do this please follow the next steps: 1) Send the audio outputs of your Synth or/and module into Logic using your mixer or the direct inputs of your audio card. 2) Put an audio object (Arrange track assigned to a Track object, or a Track object in the Audio mixer) in record mode to record the incoming signal. Then, in the Arrange window, select the MIDI track(s) that you want to record, and mute all the unwanted ones. Press Record, and you will be converting the MIDI data into audio.
Other method: in the Audio mixer, set up Input objects, whose input matches the soundcard inputs to which your synths are connected. Now when you play back the song, the MIDI is sent to the synths, which generate sound, which in turn is fed back into Logic “live”. In this case you don’t have to record the synths’ outputs first, but you can bounce the entire song directly. This of course supposes that your soundcard has enough inputs to handle the audio-output of your synths, or that you use a mixer to mix your external synths, and feed the stereo out of the mixer into a single input pair of your soundcard.